Stents are devices that are inserted into a vessel or passage to keep the lumen open and prevent closure due to a stricture, external compression, or internal obstruction. In particular, stents are commonly used to keep blood vessels open in the coronary arteries and they are frequently inserted into the ureters to maintain drainage from the kidneys, the bile duct for pancreatic cancer or cholangiocarcinoma or the esophagus for strictures or cancer.
In particular, airway stents are principally utilized for four indications, namely: (1) re-establishment of airway patency due to extrinsic tracheobronchial compression from either mass or lymph nodes; (2) re-establishment of airway patency due to intrinsic tracheobronchial obstruction from malignant or benign disease; (3) to cover a fistula track secondary to tracheo-esophageal fistula; and/or (4) to maintain airway patency in patients with tracheobronchial malacia. There are currently two basic types of stents available for some but not all of these indications: polymer or metal.
In 1990 a silicone stent developed by Dumon was reported upon in the medical literature, it is currently the most widely utilized stent in the world. The primary advantage of the silicone stent is its removability. However, this stent must be placed through a rigid metal tube (rigid bronchoscopy), in an operating suite, under general anesthesia, which increases the cost of the procedure and potentially places the patient at greater risk for complications. This technique requires extensive training and is only performed at specialized centers. The Dumon stent is thick-walled, which increases airway resistance and decreases mucociliary clearance. This problem leads to mucous impaction and tracheobronchitis. Additionally, these polymer stents are of fixed luminal diameter and do not self expand to meet the changing contour of the airway. This leads to a problem with stent migration. The cylindrical tube design does not conform to curved or conical airway anatomy and they also cause the formation of granulation tissue, which results from airway irritation.
In light of these disadvantages, and at the expense of removability, industry has moved away from the polymer stent in favor of the self-expanding metal stent. The two most widely used are the ultraflex and wall stent, which have shape memory characteristics. They are self-expanding and can be placed through a flexible bronchoscope, under conscious sedation, using local anesthesia in an outpatient setting. They have sufficient wall to lumen ratio, minimal interference with mucociliary clearance and conform to difficult airway anatomy. Unfortunately, after approximately six weeks, the wire mesh in these stents becomes epithelialized, thus making removal difficult, if not impossible.
Rejection of the stent can occur with severe airway irritation and tracheobronchitis that is impossible to treat because the nidus for the infection is the metal, which cannot be removed. Because of the inability to remove these stents, they are indicated only as a last resort for benign disease. Additionally, these stents can be challenging to deploy because they can elongate or foreshorten, depending upon the diameter of the airway.
An additional disadvantage of conventional metal stents is that they can migrate, like polymer stents, since the axial working length of these stents varies when the stent is radially compressed. Attempts have been made to address this problem by providing a stent that is comprised of knit layers of metal to form a wire mesh with peristaltic capabilities. Unfortunately, by preparing a stent from twisted wire portions, the likelihood of tissue aggravation increases because the weaved loops of the stent dislocate when subjected to radial compression. Moreover, for certain stents, sharp edges exist at the final loop ends.
As a result, physicians have the intractable dilemma of having to decide whether the patient should undergo the intricate procedure to receive the removable polymer stent, which can migrate and/or cause granulation tissue formation, and is subject to recurrent infections. Though the metal stent is easier to implant, the risk of infection and granulation tissue formation is not reduced because the stents become epithelialized and, therefore, impossible to remove.
An additional limitation of conventional stents is the inability to adapt a single design to diverse locations of the patient's anatomy. For example, as a result of differences in topology, physicians are generally required to find different devices from different manufacturers to address conditions in varying parts of the patient's anatomy. A uniform design and method of implantation while still allowing for the shape and resiliency modification necessary to accommodate the intricacies of various lumens throughout the body would be advantageous. Therefore, there is a need for a uniform prosthesis or family of related devices that can address various anatomical challenges while allowing the physician to develop a comfort level with a particular product design and implantation method.
Therefore, there also remains an existing need for a prosthesis that is; removable, prevents epithelialization thereof, does not migrate, and is suitably configured to minimize infections and airway irritation. This is of principal importance because in tracheobronchial stenting, unlike other lumens in the body, the airway is constantly exposed to inhaled bacteria thus increasing the risk of infection. However, there is a need for a prosthesis that carries the above advantages while being suitable for use in a wide variety of anatomic locations within a patient. The configuration must also facilitate a method of introduction that prevents the premature elongation and foreshortening of the stent while suitably engaging the desired implantation location. The stent must also retain its axial length while undergoing radial compression. Moreover, there is an existing need for a stent that has both antimicrobial and chemotherapeutic properties so that the stent can be indicated as an early stage therapy.
There is an existing need for a prosthesis that is designed to accommodate varying tissue types in lumens of the body. In particular, there is a need for a family of stents where the relative hardness/softness of regions of the stent can differ from other regions of the stent to provide additional patient comfort and resistance to radial forces. There is also an existing need for a family of stents with novel interstice configurations that facilitate flexibility, durability and/or proper installation. Presently, there is a need for a self-expanding stent have the above benefits that also defines a plurality of apertures at the termini of the stent for, inter alia, removal of the stent.